1. Field of the Invention
The invention relates to a process for the continuous monitoring of a mechanical component of an engine, in particular an important rotary component such as a rolling bearing or a gear of an aircraft engine, and involves monitoring in which the signal from one or more acceleration sensors is processed. The processing of the vibrational signal emanating from the sensor leads to the determination of one or more quantities resulting from or derived from the signal via the processing. These quantities are compared with thresholds so as to detect whether the monitored mechanical component has suffered damage.
2. Summary of the Prior Art
It is known to dispose acceleration sensors in places where these sensors can detect vibrations arising from the mechanical components to be monitored or from the machine in general, then to process the signals originating from the sensors so as to detect a significant abnormality of the signal transmitted by the sensor relative to the signal received in the absence of a fault.
Ideally, the detection of such abnormalities ought to allow a forecast of the remaining lifetime of the component being monitored before the damage becomes serious enough to lead to fracture or to a grave malfunction.
In practice, one generally does not have sufficient experience with regard to a large enough number of components to obtain statistically significant data for arriving at such a forecast.
In practice, the result of the processing is such that damage can be detected early enough so that at the time of detection, the member is still operating satisfactorily and one can be reasonably certain that it will continue to operate satisfactorily, preferably until a next periodic inspection of this member and at least until a next stopover when the member is mounted on a flying machine.
Patent applications EP 0889313 to 0889316 A2 filed on Jul. 3, 1998 disclose a good example of such methods.
In these applications, fifteen acceleration sensors and two azimuth sensors for sensing speeds of rotation are disposed at various sites close to members which together make up a block for transmitting motions to rotary members of a helicopter, in particular to the shaft of the main rotor and to the rear stabilising propeller.
All the signal processing methods described in these four applications comprise a step of acquiring the signal from an acceleration sensor, this involving a step of digitizing the signal from the sensor, carried out for example by means of a sample-and-hold circuit and an analog/digital converter.
The temporal signal thus sensed is transformed into a signal in the frequency domain, in each of the three application Ser. No. 08/89,313, ""314 and ""315. An initial sequence of values is thus obtained, each determining a vibration frequency and the amplitude associated with this frequency.
The processing described thereafter in application Ser. No. 08/89,313 provides for selecting and processing of the frequency samples so as to obtain a final sequence of samples. After returning to the time domain, an order moment of this temporal signal is calculated on the basis of the final sequence of samples and is compared with a predetermined threshold so as possibly to actuate an alarm. The method described in the ""313 application is intended to detect the occurrence of an abnormality located at the level of an exterior shaft.
The processing described thereafter in the ""314 application provides for the selecting of a determined frequency sample, the calculating of the amplitude of this sample and the comparison thereof with a reference value, the result being compared with a threshold.: This relatively crude method is intended to detect abnormalities which develop rapidly in flight.
The processing described thereafter in the ""315 application provides for the selecting of two groups of frequency samples, the calculating of the energy associated with these two groups, a calculation of the deviation in energy between the two groups and the comparison of this deviation with a threshold. This method is intended to detect a fault on a shaft comprising two gears.
The processing described in the ""316 application envisages, after the acquisition phase, a Hilbert transformation of the signal obtained, the defining of a complex number having the signal as its real part and the Hilbert transform as its imaginary part, the calculation of the phase of this complex number and of its derivative with respect to time, and lastly the comparison of this derivative with a threshold value.
In each of these four applications, ratios or variations are sought which impinge on what is called xe2x80x9can engagement frequencyxe2x80x9d or harmonics of this frequency. It is assumed that this involves the number of revolutions per second of the toothed wheel being monitored.
The reasons why, in these applications, certain quantities are tracked rather than others, are not explained, and therefore it is not known to what extent the teaching of these applications may be used in a different context from that described.
The teaching which may be gleaned from these examples is that the analysis of vibration signals from sensors placed close to rotating components may provide indications about the mechanical condition of these components. In particular, incipient cracking or a spreading crack can be pinpointed by virtue of such analysis. However, for each particular case, one needs to determine which frequencies are the ones which need to be analyzed and which out of all the processing possibilities are the most significant quantities to be monitored in order to obtain significant information about the mechanical components monitored.
The invention is intended to provide early detection of damage arising in particular in a revolving component of an engine for example a ball or roller bearing, or a gear. It is, however, possible to extend the field of the invention to a nonrevolving component, for example a fixed component such as a cowling, or to a component which is mobile in some other manner, for example a connecting-rod or a valve and its stem.
As in the prior art, the invention employs digital capture of the signal output by at least one acceleration sensor dedicated to the vibrational monitoring of the engine. This temporal signal is thereafter, as in the prior art, transformed into the frequency domain. It has been seen that in the prior art, a filtering is thereafter performed so as to select characteristic frequencies of the sought-after fault, and which in the aforementioned patent applications have been called xe2x80x9cengagement frequenciesxe2x80x9d. Processing operations are then performed on these characteristic frequencies so as to obtain quantities that can be compared with thresholds so as to draw conclusions therefrom relating to possible damage to the mechanical component tracked.
It has been found that the frequency signal obtained by transforming the temporal signal emanating from the sensor is very noisy. It has also been found that the signal corresponding to what has been called the engagement frequency reaches the sensor in a very weakened state, so that it is difficult to isolate any possible presence of this engagement frequency from the ambient noise necessarily present in an engine.
It has been further found that this xe2x80x9cengagement frequencyxe2x80x9d can be present even in the absence of any fault, merely through the fact that the teeth intermesh. What needs to be detected therefore is a significant modification of the spectrum associated with this engagement frequency, for example a modification of the number of harmonic lines of this frequency and/or of the amplitude of vibration at this engagement frequency or at its harmonics.
A hypothesis has been put forward by the inventors that this engagement frequency could be present as a modification frequency of one or more fundamental frequencies of the engine. The expression fundamental frequencies of the engine refers to the frequency of rotationxe2x80x94number of revolutions per secondxe2x80x94of the engine, or for engines having a low pressure compressor and a high pressure compressor, the frequencies of rotation of each of these bodies. In general, a fundamental frequency of an engine is a frequency of rotation of an important part, from the moment of inertia point of view, of the engine. A frequency resulting from damage will be a frequency at which this damage produces a shock. Ideally, the engagement frequency or rolling frequency would result from a succession of DIRAC pulses at this frequency.
In practice, these pulses have a less pure shape than a DIRAC pulse. The spectrum of the fault signal is therefore composed of lines at the frequency corresponding to the nature of the fault and at harmonic frequencies. The envelope of this line spectrum is determined by the shape of the pulses. It will be seen that in the case of searching for damage to a bearing race the damage frequency specific to the presence of the fault represents, per unit time, the number of occasions on which this fault is impacted by a ball or a roller of the bearing. In the case of a gear in which for example a tooth of a first wheel might be cracked or damaged, the frequency resulting from the damage would be the frequency with which this tooth comes into contact with the teeth of a second wheel meshing with the first. These examples show that, knowing the structure of the engine, its speed of rotation, and hence its instantaneous fundamental frequencies, it is possible to ascertain a damage-induced frequency emitted by the component being monitored.
The problem is therefore one of spotting the presence or the modification of characteristics of this frequency resulting from damage in a sufficiently stable and constant manner to isolate it from noise, and to conclude therefrom the presence of a fault in the monitored component, without generating false alarms. It is also important to spot this damage created frequency even though the fault at the origin of the appearance of this frequency is still at an early stage of its development. For example for a bearing, an early stage consists of the simple flaking of one of the bearing surfaces. However, at this early stage, the amplitude of the frequency or the modification of the characteristics of its associated spectrum resulting from the fault is small so that it is difficult to distinguish them from noise.
According to the invention, as in the prior art a sequence of the signals from at least one sensor of a vibratory acceleration signal will be sampled with a sampling frequency sufficient for Shannon""s condition to be fulfilled for the largest of the frequencies to be recorded. According to a characteristic of the present invention, this is followed by checking that the engine speed has remained stable throughout the duration of the sequence, so as to reject those sequences for which the relative variation in the rotational speed is above a predetermined threshold. As in the prior art, the temporal signal obtained will then be converted into a frequency signal, for example by Fourier transformation, and the fundamental frequencies of the engine will be eliminated therefrom.
Thereafter, according to an important characteristic of the invention, a correlation will be performed between the frequency signal obtained and the same signal shifted by a frequency V.
The correlation of two functions is the measure of their mutual resemblance: it is essentially a comparison process.
Numerically, the signals are represented by strings of numbers which are successive samples of a continuous waveform.
The correlation function of f(k) and g(k), the two numerical strings obtained by sampling two continuous functions f(t) and g(t), is expressed mathematically by the relation:       S    ⁡          (      k      )        =            ∫              i        =                  -          ∞                      ⁢                  f        ⁡                  (          i          )                    ·              g        ⁡                  (                      i            +            k                    )                    
with I=sample index,
and k=shift index.
When f and g are different functions, we speak of cross-correlation.
When f and g are identical functions, we speak of autocorrelation.
In the case of a frequency signal expressed in the amplitude/frequency complex plane, we speak of spectrum and of spectral coherence.
Calculation of spectral coherence between the transform in the frequency domain of the vibrational signal picked up and this same transform shifted by a frequency V makes it possible to identify the resemblances between the two transforms. Under these conditions, the noise frequencies, unstable by nature, will be eliminated so as to leave only the stable frequencies, that is to say those which result from a stable origin.
These are the fundamental frequencies, their harmonics, the linear combinations of these frequencies and any stable modulating frequencies.
The stable modulating frequencies may comprise, even in the absence of a fault, the engagement frequency or in the case of a bearing, a rolling frequency resulting from the number of balls or rollers and from the frequency of rotation of the assembly formed by these balls or rollers.
If, for example, FN1 and FN2 are the frequencies of rotation of sizeable masses of the engine, for example the rotors of a low pressure body and of a high pressure body respectively, the signal output by the sensor will include the fundamental frequencies FN1 and FN2. It could also include the harmonics of these frequencies for example 2FN1, 3FN1, 2FN2, 3FN2. Moreover if the transmission chain is such that a signal which includes the frequency FN1 passes through a vibrating member especially for example at the frequency 2FN2, one would have frequencies resulting from a modulation of one frequency by another, for example 2FN2+FN1, 2FN2xe2x88x92FN1. For these reasons, the spectrum of an engine will exhibit, for each particular case, that is to say for each type of engine and each sensor location, and also for ranges of engine speed, a characteristic line spectrum.
For one type of engine, a sliding coherence calculation, that is to say one in which the shift. V is varied step by step from frequency values resulting from combinations of fundamental values obtained over small multiples of the fundamental frequencies for example the multiples lying, limits included, between 0 and 4 and around these combinations, will make it possible to spot in respect of the engine speed investigated, for example in respect of those speeds which are used the longest during each flight, the lines normally present, that is to say in the absence of faults. The same search for lines will then need to be performed with an engine equipped with a component exhibiting at an early stage the fault which one will then seek to detect.
For this search, the shift V will be fixed at frequency values resulting from the modulation of the frequencies present during normal operation by the frequency resulting from damage or by harmonics of this frequency.
In order for the method of the invention to be of interest, the spectrum resulting from the fault must be different from the spectrum during normal operation, through the presence of a frequency which is not present with an amplitude sufficient to be detected even by means of the spectral coherence calculation. Indeed, any modification to the amplitude of this frequency by reason of the presence of the fault does not necessarily cause a significant variation in the value of the coherence peak.
On the other hand, such an amplitude modification for a frequency by reason of the fault may be detected by known methods comprising a bandpass filtering centered on the sought-after frequency and a calculation of the energy or of the amplitude of the spectrum resulting from the filtering.
To summarize, according to the invention there is provided a process for the early detection of the appearance of a fault in a component of an engine, said process comprising:
a preliminary phase of investigation using an engine of the same design to identify, for at least one operating speed of the engine, frequency spectral lines present during operation of the engine in the absence of a fault and then in the presence of a fault so as to identify specific spectral lines for the fault; and
a detection phase during which the following steps are carried out in an iterative manner in the course of operation of said engine:
acquiring a string of digital samples representative of a vibratory acceleration signal in the course of an acquisition sequence;
checking that the speed of said engine has remained stable during said acquisition sequence;
and applying to the stable sequences acquired the following processing operations in real time during operation of said engine or in delayed time:
transforming the signal acquired by temporal sampling into a frequency signal, while eliminating the fundamental frequencies of said engine and their harmonics,
performing at least one normed coherence calculation between said frequency signal and the same frequency signal shifted by a frequency value corresponding to the value of one of the specific frequencies of the fault detected during said preliminary phase,
comparing the value of a coherence peak obtained by the coherence calculation with a first threshold and storing a 1 detection value if this peak is greater than said first threshold and a 0 value if said peak is not greater than said first threshold,
summing P stored detection values and dividing the sum obtained by P to provide a detection ratio, and
deeming a fault to be present if said detection ratio is greater than or equal to a second predetermined threshold.
Preferably, the P detection values which are used to establish the detection ratio are values corresponding to P sequences chosen from among the most recent ones recorded in engine speed ranges considered to be of interest. These will not necessarily be ones which are most recent in time in an absolute manner, but ones which are most recent within the engine speed ranges judged to be of particular interest.
The number P is an integral number greater than or equal to 1.